Polyaxial screw system

ABSTRACT

A polyaxial screw system includes a drive cap having a partition wall with a first side and an opposing second side, a passage extending through the partition wall. A sleeve projects from the second side of the partition wall, the sleeve and partition wall bounding a socket. A shaft outwardly projects from the first side of the partition wall. A screw has a threaded portion and a head formed on an end thereof. The head of the screw is at least partially disposed within socket of the drive cap. A collet is at least partially disposed within the socket of the drive cap. At least a portion of a compression cap movably extends through the opening in the partition wall so that the compression cap can selectively move the collet relative to the drive cap.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/053,545, filed May 15, 2008, which application is incorporatedherein by specific reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to polyaxial screw systems that can beused for stabilizing adjacent vertebrae of the spine or other adjacentbones. The polyaxial screw system can be used with a rod clamp, plate,or other stabilizing structures.

2. The Relevant Technology

Polyaxial screws are commonly used in spinal operations for adjusting orstabilizing adjacent vertebrae. For example, in one conventionalprocedure a first polyaxial screw is screwed into a first vertebraewhile a second polyaxial screw is screwed into an adjacent secondvertebrae. A stabilizing rod is then secured between the polyaxialscrews so as to fix the adjacent vertebrae relative to each other.Polyaxial screws can be positioned on each side of each vertebrae andcan be positioned in any number of consecutive vertebrae with one ormore rods extending between the different polyaxial screws.

One conventional polyaxial screw comprises a bone screw have a U-shapedcollar pivotably mounted on the end thereof. The stabilizing rod isreceived within a U-shaped slot of the U-shaped collar and securedtherein by a set screw being threaded into the U-shaped slot of thecollar and biased against the rod. Although such systems are functional,they have certain drawbacks. For example, as a result of the collarbeing U-shaped, there is some risk that the collar will outwardly flareand thus fail as the set screw is threaded into the U-shaped slot andsecured against the rod.

An additional problem with the above conventional polyaxial screw isthat due to the specialized configuration of the U-shaped collar, thescrew can only be used directly with a stabilizing rod. In some cases,based on the situation or surgeons preference, it may be desirable touse plates or other non-rod stabilizing structures to extend between thepolyaxial screws. Accordingly, what is needed are polyaxial screwsystems that overcome some or all of the above disadvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be discussed withreference to the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope.

FIG. 1 is a perspective view of one embodiment of a polyaxial screwsystem of the present invention used with a stabilizing rod;

FIG. 2 is an exploded perspective view of the polyaxial screw systemshown in FIG. 1;

FIG. 3 is a perspective view of the bone screw shown in FIG. 2;

FIG. 4 is a top perspective view of the collet shown in FIG. 2;

FIG. 5 is a bottom perspective view of the collet shown in FIG. 4;

FIG. 6 is a cross sectional side view of the collet shown in FIG. 4;

FIG. 7 is a cross sectional side view of the drive body shown in FIG. 2;

FIG. 8 is a bottom perspective view of the drive body shown in FIG. 7;

FIG. 9 is a top perspective view of the drive body shown in FIG. 7;

FIG. 10 is a bottom perspective view of the compression cap shown inFIG. 2;

FIG. 11 is a perspective view of the rod clamp shown in FIG. 2;

FIG. 12 is a disassembled perspective view of the rod clamp shown inFIG. 11;

FIG. 13 is a cross sectional side view of the collet shown in FIG. 6received within the socket of the drive body shown in FIG. 7;

FIG. 14 is a cross sectional side view of the polyaxial screw systemshown in FIG. 1 with the drive body in an unlocked position;

FIG. 15 is a cross section side view of the polyaxial screw system shownin FIG. 14 with the drive body in a locked position; and

FIG. 16 is an alternative embodiment of the polyaxial screw system shownin FIG. 1 with the rod clamp being replaced by a plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Depicted in FIG. 1 is one embodiment of a polyaxial screw system 10incorporating features of the present invention. In one embodimentpolyaxial screw system can be used for stabilizing adjacent vertebrae ofa spine as part of a procedure for fusing together the adjacentvertebrae. Polyaxial screw system 10 can also be used for stabilizing aseries of consecutive vertebrae for manipulation of the spine to correctspinal deformities such as scoliosis. It is appreciated that polyaxialscrew system 10 and/or discrete elements thereof can also be used inother procedures for anchoring, manipulating, and/or stabilizingportions of the spine or other bones.

As depicted in FIG. 2, polyaxial screw system 10 comprises an elongatedscrew 12, a collet 14, a drive body 16, a compression cap 18, astabilizing member, which in the depicted embodiment includes a rodclamp 20, and a fastener 21. The above identified components and theirrelative interaction will now be discussed in greater detail.

Turning to FIG. 3, screw 12 comprises an elongated shaft 22 having afirst end 24 and an opposing second end 26. One or more threads 28helically encircle and radially outwardly project from shaft 22 alongthe length thereof. The one or more threads 28 can have a variety ofdifferent pitches and configurations, and, if desired, can be selftapping. Although not required, second end 26 terminates at a taperedtip 29 having a substantially conical configuration for ease inpenetration into a bone or predrilled hole. In contrast to second end26, an enlarged head 30 is disposed at first end 24 of shaft 22. Shaft22 has a central longitudinal axis 32 which centrally extends throughtip 29 and head 30.

Although not required, in the embodiment depicted head 30 has asubstantially spherical configuration. An engagement slot 34 is formedon head 30. Engagement slot 34 comprises a pair of opposing side walls36 and 38 that are generally disposed and parallel plains and thatextend to a floor 40 and a back wall 42. As depicted in FIG. 15, backwall 42 typically intersects with floor 40 at a right angle while backwall 42 is disposed generally parallel to central longitudinal axis 32at a distance space apart therefrom. It is appreciated that slot 34 canhave a variety of different configurations and merely needs to be sized,shaped, and oriented to permit the desired pivoting of drive body 16 androtation of screw 12 as will be discussed below in greater detail.

Depicted in FIGS. 4-6, collet 14 comprises a substantially disk-shapedbody 44 having a side wall 46 that extends between a first end wall 48and an opposing second end wall 50. End walls 48 and 50 are disposed insubstantially parallel planes. Side wall 46 has an exterior surface 62that comprises an upper face 64 disposed toward first end wall 48 thatencircles body 44 having a substantially cylindrical configuration and abeveled face 66 disposed toward second end wall 50 that encircles body44 and slopes inwardly.

A recessed pocket 52 is centrally formed on second end wall 50. Pocket52 is bounded by an interior surface 53 and has a configurationsubstantially complimentary to head 30 of screw 12. Pocket 52 isundercut having a configuration that is slightly more thansemispherical. As a result, body 44 comprises a substantially annularlip 54 that radially, inwardly projects into pocket 52 at or adjacent tosecond end wall 50 so as to form a constricted opening 56 to pocket 52.In this configuration, head 30 of screw 12 can be snap-fit into pocket52 with lip 54 preventing unwanted separation between screw 12 andcollet 14 (FIG. 14). Due to the complimentary configuration betweenpocket 52 and head 30, collet 14 can freely pivot on head 30. Inalternative embodiments, it is appreciated that lip 54 need notcompletely encircle opening 56 but can comprise a plurality of spacedapart sections that capture head 30 within pocket 52. Furthermore, head30 need not be completely spherical but can be truncated or have arecess formed on the top end thereof.

As also depicted in FIGS. 4-6, an opening 58 is centrally formed onfirst end wall 48 so as to extend to recessed pocket 52. Likewise, achannel 60 extends through side wall 46, first end wall 48, and secondend wall 50 so as to communicate with pocket 52 and openings 56 and 58.As a result of channel 50, collet 14 has a substantially C-shapedconfiguration when viewed in a top or bottom plan view. A plurality ofradially spaced apart slots 62, extend through first end wall 48beginning at opening 56 and extending out toward side wall 46. Slot 62continue along interior surface 53 of pocket 52 down to second end wall50. In the embodiment depicted, slots 62 are recessed on interiorsurface 53 of side wall 46 but do not extend through side wall 46. Inalternative embodiments, slots 62 can extend through a portion of sidewall 46. Slots 62 act in conjunction with channel 60 to enable collet 14to radially, outwardly dilate and also radially, inwardly constrict whenradial inward and outward forces are applied collet 14. For example,collet 14 resiliently, outwardly expands or dilates when head 30 ofscrew 12 is snapped-fit within pocket 52 and, as will be discussed belowin greater detail, resiliently, inwardly constricts when collet 14 isused to lock screw 12 relative to drive body 16. It is appreciated thatother configurations and orientations of slots 62 and/or channel 60 canbe formed on collet 14 to enable collet 14 to selectively dilate andconstrict.

Turning to FIG. 7, drive body 16 comprises a drive cap 70 having a shaft72 projecting therefrom. Drive cap 70 comprises a partition wall 74having a first side 76 and an opposing second side 78. A tubular sleeve80 outwardly projects from second side 78 of partition wall 74 andterminates at an annular terminal end face 75. Sleeve 80 has an interiorsurface 82 that combines with second side 78 of partition wall 74 tobound a socket 88. Interior surface 82 has a first end 84 at whichpartition wall 74 is formed and an opposing second end 86 at which anopening 90 is formed for socket 88.

In the embodiment depicted, interior surface 82 at first end 84 has asubstantially cylindrical configuration that encircles socket 88. Secondend 86 of interior surface 82 radially inwardly constricts towardterminal end face 75 so as to have a substantially frustoconicalconfiguration that encircles socket 88. As a result, the diameter atsecond end 86, and particularly the diameter at opening 90, is smallerthan the diameter at first end 84. A passage 91 transversely extendsthrough sleeve 80 at second end 86. As shown in FIGS. 7 and 8, a pin 92is secured within passage 91 so that a free end 94 of pin 92 projectsinto socket 88 as second end 86. Pin 92 can be secured within passage 91by welding, press fit, adhesive, or other conventional techniques.

As shown in FIGS. 7 and 9, an annular recess 98 having a floor 102 isformed on first side 76 of partition wall 74 and encircles shaft 72. Aplurality of spaced apart passageways 100A-C pass through partition wall74 by extending from floor 102 of annular recess 98 to socket 88.Passageway 110A is generally illustrated in FIG. 7 as being verticallyaligned with pin 92 for ease in illustration of the different parts. Inpractice, however, for reasons as will be discussed below in greater,passageway 110A-C are typically off set from pin 92 as shown in FIG. 9.Drive cap 70 also has an exterior surface 96 that encircles drive cap 70and extends from first side 76 of partition wall 74 to terminal end face75. A portion of exterior surface 96 toward first side 76 has apolygonal or irregular configuration. As a result, a tool, such as inthe form of a socket or wrench can be used to engage drive cap 70 tofacilitate rotation thereof. In alternative embodiments, it isappreciated that various slots, openings, recesses and/or other contourscan be formed on drive cap 70 to enable engagement with a speciallydesigned tool to facilitate rotation of cap 70.

Shaft 72 centrally projects from partition wall 74 and extends between afirst end 104 and an opposing second end 106. As shown in FIG. 7, shaft72 has one or more helical threads 102 encircling and formed along thelength of shaft 72 for threaded engagement with fastener 21. A centrallongitudinal axis 102 centrally extends through drive body 16 so as tocentrally extend through shaft 72.

Returning to FIG. 2, compression cap 18 comprises a base 110 having aplurality of arms 112A-C projecting therefrom. More specifically, asdepicted in FIG. 10, base 110 has a substantially circular disk shapedconfiguration with a side wall 114 extending between a top surface 116and an opposing bottom surface 118. An opening 120 centrally extendsbetween top surface 116 and bottom surface 118 and is sized to permitshaft 72 of drive body 16 to pass therethrough. Base 110 is configuredto be received within recess 98 of drive body 16 (FIG. 15). Likewise,each of arms 112A-C are configured to be received within correspondingpassageways 100A-C on drive body 16 when base 110 is received withinrecess 98. Arms 112A-C each terminate at a free end 113 have a lengthlonger than the length of passageways 100A-C. As a result, free end 113of each arm 112A-C can pass down through passageways 100A-C and projectinto socket 88 (FIG. 15).

In alternative embodiments, it is appreciated that the plurality of arms112A-C can be replaced with one larger arm, two spaced apart arms, orfour or more arms. Passageways 100A-C would be modified accordingly.Likewise, base 110 can have a substantially C-shaped configuration orcompression cap 18 can be divided into two or more separate parts, eachpart comprising a portion of base 110 and having one or more armsprojecting therefrom. In still other embodiments, arms 112A-C can beseparate from base 110 or base 110 can be eliminated and rod clamp 20can be used to press arms 112A-C in passageways 100A-C.

Turning to FIGS. 11 and 12, rod clamp 20 comprises a first clamp arm 130and a second clamp arm 132. First clamp arm 130 comprises a plate 134having a top surface 136 and an opposing bottom surface 137 that extendbetween a first end 138 and an opposing second end 139. An elongatedchannel 140 extends through plate 134 between opposing surfaces 136 and137 and longitudinally extends along the length of plate 134. Channel140 is configured to enable shaft 72 of drive body 16 to passtherethrough. Two spaced apart arms 142 and 144 each having a generallyU-shaped configuration down and outwardly project from second end 139 ofplate 134.

Similarly, second clamp arm 132 comprises a plate 150 having a topsurface 152 and an opposing bottom surface 154 that extend between afirst end 156 and an opposing second end 158. Again, an elongatedchannel 160 extends through plate 150 between opposing surfaces 152 and154 and longitudinally extends along the length of plate 150. Channel160 is also configured to enable shaft 72 of drive body 16 to passtherethrough.

A singular arm 162 having a substantially U-shaped configuration andinverted relative to arms 142 and 144 projects up and outwardly fromsecond end 158 of plate 150. Clamp arms 130 and 132 are designed to nesttogether as depicted in FIG. 11 with arm 162 being received between arms142 and 144. In this configuration, arms 162, 142, and 144 combine toform a locking jaw 164 having a mouth 166 formed therebetween. Jaw 164is configured to lock a rod 168 (FIG. 1) within mouth 166. That is, byseparating first ends 138 and 156 of clamp arms 130 and 132 whileretaining second ends 139 and 158 together, mouth 166 of jaw 164 iswidened in the same fashion that shears of a scissor are separated. Oncein this enlarged position, rod 168 can be positioned between arms 142,144 and arm 162. Clamp arms 130 and 132 can then be pressed backtogether causing arms 142, 144 and arm 162 to bias or clamp on opposingsides of rod 168 and thereby lock rod 168 in place. The U-shaped contourof arms 142, 144 and 162 can be generally complimentarily to thediameter of rod 168 to facilitate capturing of rod 168 between the arms.

Returning back to FIG. 2, in the embodiment depicted fastener 21comprises a threaded nut having a side wall 172 that extends between atop surface 174 and an opposing bottom surface 176. Side wall 172typically has a polygonal, oval or some other irregular configurationthat would enable a tool to selectively engage and rotate fastener 21. Athreaded opening 178 extends through fastener 21 from top surface 174 tobottom surface 176. Opening 178 is configured to threadedly engage shaft72 so that fastener 178 can selectively advance along the length ofshaft 72 by being rotated thereon.

Assembly and operation of polyaxial screw system 10 will now bediscussed. Initially, as depicted in FIG. 13, collet 14 is radiallyinwardly compressed and then inserted into socket 88 of drive body 16 sothat pin 92 is received within channel 60 of collet 14. Once collet 14passes through constricted opening 90 of socket 88, collet 14resiliently rebounds to its original configuration. In its originalconfiguration, collet 14 has a diameter larger than constricted mouth 90so that collet 14 cannot unintentionally fall out of socket 88. However,first end 84 of socket 88 is slightly larger than collet 14 so thatcollet 14 has some free play within socket 88. As a result of pin 92begin received within channel 60 of collet 14, collet 14 is preventedfrom rotating within socket 88 about axis 102.

Next, as depicted in FIG. 14, head 30 of screw 12 is snap-fit withinpocket 52 of collet 14 so that pin 92 is received within slot 34 of head30. Again, the annular undercut lip 54 of collet 14 prevents head 30from unintentionally disengaging from collet 14. However, as a result ofhead 30 having a complementarily configuration to pocket 54, head 30 isfree to pivot within collet 14. Expressed in other terms, collet 14 anddrive body 16 can be pivoted in any direction on head 30 of screw 12when screw 12 is fixedly mounted to a bone. Although drive body 16 canbe pivoted relative to screw 12, as a result of pin 92 being receivedwithin slot 34, drive body 16 cannot rotate about axis 102 independentof screw 12. Rather, rotation of drive body 16 causes pin 92 to biasagainst side wall 36 or 38 of slot 34 (FIG. 3) which in turn facilitatesconcurrent rotation of screw 12.

Accordingly, by using a tool that engages the exterior surface 96 ofdrive body 16, drive body 16 can be selectively rotated which in turnrotates screw 12 for driving screw 12 into a bone. Again, even afterscrew 12 is fixed within a bone, drive body 16 and thus shaft 72 can befreely pivoted in any direction on head 30 of screw 12. Once screw 12 isfixed at a desired location, such as on a vertebra or other bone,compression cap 18 can be advanced over shaft 72 so that arms 112A-C ofcompression cap 18 (FIG. 9) are received within correspondingpassageways 100A-C of drive body 16. Coupled clamp arms 130 and 132 canthen be advanced onto shaft 72 by advancing shaft 72 up through alignedchannels 140 and 160. Fastener 21 is then advanced onto the end of shaft72. Prior to tightening fastener 21 against clamp 20, clamp arms 130 and132 are separated as previously discussed and a rod 168 (FIG. 1) isreceived within the open jaw 164. The ability to pivot drive body 16relative to screw 12 facilitates alignment of rod 168 with jaw 164. Thisis particularly true when rod 168 is being connected to a seriespolyaxial screw systems 10 that are fixed in a series, such as onsequential vertebra of a spine.

Once rod 168 is in the desired position and drive body 16 isappropriately oriented relative to screw 12, fastener 21 is selectivelyadvanced down along shaft 72. As fastener 21 is advanced down shaft 72,base 110 of compression cap 18 and opposing arms of rod clamp 20 arecompressed between fastener 21 and drive cap 70 of drive body 16. As aresult of this compression, clamp arms 130 and 132 are pressed togetherso as to securely engage rod 168 while arms 112A-C are advanced intosocket 88 of drive body 16 through passageways 100A-C.

Turning to FIG. 15, as arms 112A-C are advanced into socket 88, free end113 of each arm 112A-C bias against the top of collet 14 and pushescollet 14 towards opening 90 of socket 88. However, because opening 90of socket 88 is constricted relative to the size of collet 14, beveledface 66 of collet 14 securely wedges against tapered second end 86 ofdrive cap 70 and causes collet 14 to radially inwardly compress againsthead 30 of screw 12. As a result, drive body 16, collet 14, and screw 12are fixed and locked relative to each other. In turn, by looseningfastener 21 on shaft 72, drive body 16 can again be pivoted on head 30of screw 12. As previously discussed, passageways 100A-C (FIG. 9) ofcompression cap 18 are typically rotationally offset from verticalalignment with pin 92. This ensures that arms 112A-C (FIG. 10) that passdown through passageways 100A-C each bias against the top of collet 14as opposed to one of arms 112A-C aligning with channel 60 of collet 14and thus not bias against the top of collet 14.

In one embodiment of the present invention, means are provided forsecuring compression cap 18 to drive body 16 independent of fastener 21.By way of example, prior to positioning collet 14 into socket 88,compression cap 18 can be mounted on drive body 16 with arms 112A-C ofcompression cap 18 (FIG. 9) being received within correspondingpassageways 100A-C of drive body 16 so that the free ends 113 of arms112A-C are received within socket 88. In this configuration, free end113 of one or more of arms 112A-C can be slightly bent so that arms112A-C can still slide within passageways 100A-C but cannot completelypass out through passageways 100A-C. As a result, compression cap 18 issecured to drive body 16 at the time that screw 12 is initially insertedinto the bone and does not need to be separately mounted at a later timewith rod clamp 20. In other examples of the means for securing, one ormore of free ends 113 can be compressed using a press so as to flare outwithin socket 88. Alternatively, a spot weld or drop of adhesive can beformed at one or more of free ends 113 so as to increase the size of theone or more free ends 113 and thereby prevent free ends 113 from passingback out through passageways 100A-C. Other techniques can also be used.

In still other methods of use, it is appreciated that compression cap 18can be positioned in its removable state on drive body 16 prior to thetime of mounting screw 12 to a bone as long as care is taken to ensurethat compression cap 18 does not fall off prior to attaching rod clamp20 and fastener 21. In yet other methods of use, compression cap 18 canbe removably positioned on drive body 16 following which fastener 21 orsome other temporary fastener is secured to shaft 72 or is otherwiseused to secure compression cap 18 to drive body 16. Screw 12 can then besecured to a bone without the risk of separation of compression cap 18.Once screw 12 is secured in place, fastener 21 or the removable fastenercan be removed following which rod clamp 20 is mounted on shaft 72 andfastener 21 is secured on shaft 72 over rod clamp 20. In essence,depending on the type of tool used to drive screw 12 into the bone, allor some of compression cap 18, rod clamp 20 and fastener 21 can bemounted on shaft 72 before or after securing screw 12 to the bone.

It is appreciated that rod 168 will typically be connected to at leastone other polyaxial screw system 10 or other type of anchor so as to fixthe adjacent vertebra or other adjacent bones relative to each other. Itis also appreciated that rod clamp 20 is only one example of astabilizing structure that can be used with the remainder of polyaxialscrew system 10. For example, any other type of rod clamp or mechanismfor securing rod 168 that can be secured on shaft 72 can also be used inthe present invention. In yet other embodiments, rod clamp 20 can beeliminated and replaced with other types of stabilizing structures thatextend between adjacent polyaxial screws. By way of example and not bylimitation, depicted in FIG. 16 is one embodiment of a polyaxial screwsystem 10A. Polyaxial screw system 10A is identical to screw system 10except that rod clamp 20 has been replaced with a stabilizing structurethat comprises plate 190.

Plate 190 comprises an elongated body 192 having a top surface 194 andan opposing bottom surface 196 that extend between opposing roundedends. An elongated slot 198 extends between opposing surfaces 194 and196 and extends along the length of body 192. Slot 198 is configured sothat shaft 72 can be passed therethrough and so that plate 190 can bepressed between fastener 21 and compression cap 18 or drive cap 70 ofdrive body 16. In alternative embodiments, it is appreciate that plate190 can have a variety of different configurations and can have aplurality of spaced apart slots of openings for engaging with a numberof different polyaxial screws.

Different embodiments of the present invention have a number of uniquefeatures over conventional systems. For example, as a result of usingshaft 72 with fastener 21 that completely encircles and screws down onshaft 72, this design has improved mechanical and structural benefitsover the prior art polyaxial screws having a U-shaped collar. Inaddition, as a result of the use of shaft 72, a number of differenttypes of plates, clamps, and other types of connectors and stabilizingstructures can be easily used with the inventive polyaxial screwsystems. In addition to the foregoing, the inventive systems are alsoeasily mountable and adjustable. Other advantages are also provided bydifferent embodiments of the present invention.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A polyaxial screw system comprising: a drive cap comprising: apartition wall having a first side and an opposing second side, apassage extending through the partition wall between the opposing sides;a sleeve projecting from the second side of the partition wall, thesleeve and partition wall at least partially bounding a socket; a shaftoutwardly projecting from the first side of the partition wall; a screwhaving a threaded portion and a head formed on an end thereof, the headof the screw being at least partially disposed within socket of thedrive cap; a collet at least partially disposed within the socket of thedrive cap, at least a portion of the collet being positioned betweendrive cap and the head of the screw; and a compression cap, at least aportion of the compression cap movably extending through the opening inthe partition wall so that the compression cap can selectively move thecollet relative to the drive cap.
 2. The polyaxial screw system asrecited in claim 1, wherein the shaft is substantially cylindrical. 3.The polyaxial screw system as recited in claim 1, wherein the shaftcentrally projects from the partition wall.
 4. The polyaxial screwsystem as recited in claim 1, further comprising a fastener adapted toselectively couple with the shaft.
 5. The polyaxial screw system asrecited in claim 4, wherein the shaft is threaded and the fastenercomprises a nut configured to threadedly engage with the shaft.
 6. Thepolyaxial screw system as recited in claim 1, further comprising: thehead of the screw having a slot formed thereon; and a pin projectingfrom the drive cap into the slot such that rotation of the drive capfacilitates rotation of the screw.
 7. The polyaxial screw system asrecited in claim 1, wherein the collet has a channel formed thereon, thepin extending through the channel of the collet.
 8. The polyaxial screwsystem as recited in claim 1, wherein the sleeve of the drive cap has aninterior surface extending between a first end having the partition wallformed thereat and an opposing second end, the second end bounding anopening for the socket, wherein the interior surface of the sleeveradially inwardly tapers at the second end thereof.
 9. The polyaxialscrew system as recited in claim 8, wherein the collet has a side wallextending between a first end face and an opposing second end face, thesecond end face having a pocket formed thereat, at least a portion ofthe head of the screw being received within the pocket of the collet.10. The polyaxial screw system as recited in claim 9, wherein at least aportion of the side wall of the collet has an outside diameter that islarger than an inside diameter of the interior surface of the sleeve atthe second end thereof.
 11. The polyaxial screw system as recited inclaim 9, wherein the compression cap comprises: a base positionedadjacent to the first side of the partition wall of the drive cap; andan arm projecting from the base and extending through the passage of thedrive cap.
 12. The polyaxial screw system as recited in claim 9, whereinthe collet further comprises: an opening formed on the first end faceand communicating with the pocket; and a channel extending through theside wall of the collet so as to communicate with the opening formed onthe first end face and the pocket formed on the second end face.
 13. Thepolyaxial screw system as recited in claim 1, further comprising: aplurality of passages extending through the partition wall between theopposing sides thereof, and the compression cap comprises: a basepositioned adjacent to the first side of the partition wall; and aplurality of arms projecting from the base, each arm extending through acorresponding one of the plurality of passages extending through thepartition wall.
 14. The polyaxial screw system as recited in claim 4,further comprising a stabilizing structure mounted on the shaft betweenthe fastener and compression cap.
 15. The polyaxial screw system asrecited in claim 14, wherein the retaining structure comprises a rodclamp or a plate.
 16. The polyaxial screw system as recited in claim 1,wherein the collet is selectively movable between a first positionwherein the drive cap can freely pivot relative to the head of the screwand a second position wherein the collet is radially inwardly compressedby the drive cap which locks movement of the drive cap relative to thescrew
 17. A method for mounting a polyaxial screw into a bone, themethod comprising: rotating a drive cap so that a screw pivotablymounted within a socket of the drive cap is screwed into a bone, acollet being positioned between the screw and the drive cap; positioninga stabilizing structure and a fastener onto a shaft projecting from thedrive cap; and advancing the fastener along the shaft so that theretention member pushes a portion of a compression cap against thecollet which in turn wedges the collet within the socket of the drivecap and causes the collet to bias against a portion of the screw,thereby locking the position of the screw, the collet and the drive caprelative to each other.
 18. The method as recited in claim 17, whereinthe step of positioning the stabilizing structure and the fastener ontothe shaft occurs either before or after the step of rotating the drivecap.
 19. The method as recited in claim 17, further comprises securing arod to the stabilizing member.
 20. The method as recited in claim 17,wherein the step of advancing the fastener comprises screwing thefastener onto the shaft so that the fastener pushes against thestabilizing member.
 21. The method as recited in claim 17, wherein thestep of positioning the stabilizing structure and the fastener onto theshaft comprises advancing the shaft through an opening in a plate.